There are numerous references relating to ways of applying hot-melt adhesives, i.e. adhesives being applied at raised temperatures in their molten state, to substrates. Examples are SE 374,489; GB 2,134,420 or EP 745,433. In these and other prior art references adhesive application techniques are described where adhesives are provided onto substrates by printing, spraying, slot- or curtain coating and the like.
Recent developments in adhesive formulation technology have resulted in more and more hot-melt adhesives, which contain significant amounts of relatively volatile materials. Examples are hot-melt adhesives containing superabsorbents, in particular materials with high absorption capacity towards aqueous fluids. These superabsorbents, typically present in the adhesive in particle form, are hygroscopic and therefore tend to accumulate water during storage of the adhesive from e.g. environmental air humidity. When such adhesives containing superabsorbent material are molten at raised temperatures for application onto absorbent articles, volatile materials that typically have boiling points lower than the application temperature of the hot-melt adhesive, in this example water, tend to evaporate and to form gas bubbles in the adhesive. These gas bubbles, however, are detrimental upon application of the adhesive. Usually, for very tightly controlled application of the adhesive, which is an absolute requirement in high speed manufacturing processes like those of absorbent articles such as diapers or feminine hygiene articles, the adhesive is delivered to the applicator means through volumetric pumps. These pumps supply a defined amount of adhesive per every revolution. However, if there are gas bubbles present in the adhesive, the actual amount of adhesive supplied by the volumetric pumps will be uneven and thus uncontrollable due to the high variation of density of the bubbles-containing adhesive. This then results in quality issues with the so-produced absorbent articles due to uneven adhesive application resulting in uneven bonding between their components. This ultimately might affect the overall absorbency capabilities of the absorbent article. Therefore conventional adhesive application methods all fail to provide the required control of the adhesive application for those particular adhesives.
It has also been observed that the particle size of the superabsorbent particles suspended in the hot melt adhesive has a major influence on this problem. The smaller the superabsorbent particles are the higher is the amount of gas bubbles generated because of the higher effective particle surface.
In view of the prior art cited it is the object of the present invention to provide a method for applying hot-melt adhesives containing volatile material, said volatile material evaporating at the application temperature of the adhesive, which method allows very constant pumping of the amount of molten hot-melt adhesive from a storage tank per time. In other words, a method for applying volatiles-containing hot-melt adhesives is needed, which allows to pump molten hot melt adhesive at a constant mass flow rate over a certain temperature range.